1. Field of the Invention.
The present invention relates to a process for weather control or modification and aerosol compositions useful therein. More specifically, the present invention relates to an improved method and compositions for the precipitation of atmospheric water by means of multicomponent aerosols.
2. Prior Art.
In both the patent and scientific literature, a number of weather modification methods have been reported, consisting in the seeding of moist atmosphere by aerosols of different chemical composition, both organic and inorganic. The purpose of the seeding is hail suppression, rain regulation or fog precipitation.
In general, aerosols act as active centers of heterogeneous nucleation of atmospheric water, causing a local drop in water vapor pressure around them, which leads to a continuous growth of water droplets or ice crystals. The number of active centers of nucleation developed by dispersion of a nucleant mass unit is used as a measure of its weather modification effectiveness.
The highest known effectiveness has been achieved by the use of silver iodide as a nucleant. Although silver iodide is a rather expensive chemical, if the total costs of dispersion into atmosphere of different nucleants are compared, the weather modification by silver iodide appears to be the most economical.
For weather modification purposes, silver iodide aerosol is developed by burning liquid solutions or solid pyrotechnical mixtures having silver iodide as a constituent. The mixtures generating pure silver iodide aerosol are generally much less effective than those generating composite aerosols.
Pure silver iodide aerosol is practically ineffective above -5.degree. C. Burkhardt, et al. (U.S. Pat. No. 3,915,379) have reported that two-component aerosols of the composition AgI-MI, where M is an alkali metal, show markedly more pronounced nucleation ability than that of pure silver iodide. DeMott, et al. (P.J. DeMott, W.G. Finnegan and L.O. Grant, J. Clim. Appl. Met., 22 (1983) 1190) have shown that the effectiveness of an aerosol developed by burning an acetone solution is considerably improved when the solution contains additives which result in an aerosol which is a mixture AgI, AgCl, and NaCl. However, the improvement was realized only at temperatures below -10.degree. C., while at -5.degree. C., the aerosol is practically ineffective.
Solid pyrotechnic mixtures based on silver iodide as reported in patent and other literature, as compared to liquid solutions, generate aerosols of an improved effectiveness in the vicinity of a threshold temperature of -5.degree. C., thus providing a more uniform nucleation ability within the temperature range of interest from the standpoint of weather modification. An additional advantage of solid mixtures is a wider variety of ways of dispersing them into clouds, such as by means of airplanes, rockets, artillery shells and ground-placed generators.
Increasing of the yield of active nuclei per mass unit of silver iodide, the shift of a threshold temperature as near as possible to 0.degree. C., and the adjustment of dispersion methods to meet the requirements of weather modification, is a never ending task in the field of weather modification.